1. Field of the Invention
This invention relates to implanted medical devices and is more particularly directed to a tachyarrhythmia detection system for incorporation into a pacemaker.
2. Description of the Prior Art
A variety of disease mechanisms may result in cardiac dysrhythmias. Typically, these dysrhythmias are characterized by electrical instability in the cardiac tissue which results in abnormal mechanical activity of the heart. The result of a dysrhythmia is typically the reduction in the rate at which oxygenated blood is circulated throughout the body. This parameter is called the cardiac output. If this loss of cardiac output results from a heartbeat slower than a normal heartbeat responding to the same physiological demand, the dysrhythmia is called a bradycardia. In contrast, an abnormally rapid beating of the heart which also results in reduced cardiac output is generically termed a pathologic tachycardia.
Early examples of pacemakers such as that taught by U.S. Pat. No. 3,478,746 to Greatbatch have been widely accepted as an appropriate therapy for bradycardias. These demand-type pacemakers provide a stimulus to the cardiac tissue through a catheter if no naturally occurring cardiac activity is sensed within a preset time period referred to as the escape interval. Consequently, stimulating pulses are supplied to the heart only when the intrinsic heart rate drops below a preset minimum rate corresponding to the escape interval.
Examples of prior art tachycardia treatment devices include the device taught by U.S. Pat. Nos. 3,698,398 and 3,693,627 to Berkovits. The teaching of these patents is directed primarily to the therapeutic stimulation applied to the heart after the detection of the tachyarrhythmia and each of these patents teach the use of a rate detection system for invoking the tachyarrhythmia treatment. Reliance on the use of detected depolarization rates of the cardiac tissue have been proposed in more recent examples of implantable tachyarrhythmia pacers including that taught by U.S. Pat. No. 4,181,133 to Kolenik, et al.
There are a number of problems associated with the use of these prior art rate detection systems for the detection of a pathologic tachyarrhythmia. One problem is that pathologic tachyarrhythmias can have rates within the range shared by normal heart activity. In effect, there is an overlap between physiologically normal and pathologic heart rates which reduces the effectiveness of a simple rate detection algorithm since it cannot reliably distinguish between the normal and the pathologic condition on rate data alone.
For these reasons, the rate threshold types of tachyarrhythmia detectors have proven inadequate to distinguish the pathologic tachyarrhythmia from related cardiac rhythms which do not require therapy. Consequently, pacers which rely on rate detectors alone have not been widely accepted for the treatment of tachyarrhythmia.